Abstract: Focal depth plays an important role in optical storage systems, and high focal depth makes the focusing servo system easy to track the disk surface. The effect of the pure-phase diaphragm on the axial light-intensity distribution and focal depth was investigated in low-and high-numerical aperture systems by means of scalar diffractive theory and vector diffractive theory, respectively. The results show that the proposed phase diaphragm affects the focal depth considerably, and the focal depth may increase by six times for certain geometrical parameters of the phase diaphragm in the systems with highnumerical aperture.